The SSL Redirect plugin is able to redirect non-secure HTTP-pages to secure HTTPS-pages, and back again. Which pages need to be served through SSL and which not, can be configured through plugin parameters: You can select manually Joomla! Menu-Items to which SSL should be applied, or you can choose Joomla! components (like the user-component) to which it should be applied, or you can add custom URLs in a textarea. The plugin takes care of the rest.
The Joomla! Global Configuration allows you to configure SSL, but if a visitor enters a Joomla! page through non-HTTPS, this visitor is not automatically redirected. Also you can’t enable SSL for just some pages, while all other pages are served through regular HTTP.
While the Apache htaccess file can be configured to contain RewriteRules for this, it is much more complicated and error-prawn. This is the main reason why the YireoTM SSL Redirect plugin was developed.